Portal 2

Though I have really enjoyed this game and makes a welcome change from my usual FPS games, I am afraid it was far to short and far to easy in comparison to the original Portal. Not much replay value as once you have solved the puzzles that is pretty much it.
Hopefully the modding fraternity will come up with some more difficult/interesting content.

Game length is a bone of contention for some; however, I'd much rather play a ten hour game that is well thought out and enjoyable with very little padding than one which drags things out with time consuming tasks (say, for example, puzzles that make you trek the length of the map doing dull fetch and carry tasks)

I've not got around to Portal 2 yet; but really enjoyed the first in the series - and as you say, the PC modding community may throw out some interesting add-ons at some point (or Valve may release additional map packs?)